Field of the Invention:
The invention relates to a sheet-fed printing press including a printing cylinder and a sensor system for monitoring a sheet run in the area of the printing cylinder. The sensor system is disposed in the sheet run area and has a transmitter and a receiver. The transmitter is disposed on one end face and the receiver is disposed on the other end face of the printing cylinder in such a manner that a viewing direction of the sensor system is at right angles to a transport direction of the sheet. The invention also relates to a method for calibrating a sensor system for monitoring the sheet run in the area of the printing cylinder of a sheet-fed printing press and a method for aligning a sensor system for monitoring the sheet run of sheets of different thickness in the area of the printing cylinder of a sheet-fed printing press.
Description of the Related Art:
The use of digital printing presses is known for the printing of sheets of paper, card, or board in small runs or with individual print motifs. When using inkjet heads for printing sheets, a respective sheet is moved through under the inkjet heads by a transport system at a minimal distance. Circulating conveyor belts, for example, constructed as suction belts, and rotating cylinders, so-called jetting cylinders, or circulating tablets as described, for example, in U.S. Pat. No. 8,579,286 B2 are known as transport systems.
In machine concepts using cylinders, as described for example, in U.S. Patent Application Publication US 2009/0284561 A1, a plurality of inkjet heads are disposed radially at a distance above a jetting cylinder. The inkjet heads print sheets moved past the printing heads at a short distance. A plurality of sheets can be sucked onto a jetting cylinder and transported simultaneously. In order to ensure a high print quality and avoid damage to the printing heads, it is important that a respective sheet lies correctly on the jetting cylinder.
In addition, it is known to monitor the sheet run and detect defective sheets or incorrectly placed sheets. In order to prevent damage to the highly sensitive printing nozzles of an inkjet head, for example due to high-standing corners, edges, or folds, the printing press is usually stopped and the defective sheet is removed.
Such a printing press is described in U.S. Patent Application Publication US 2013/0307893 A1. If a defective sheet is detected by a sensor mounted upstream of the inkjet heads, not only is the machine stopped but all of the inkjet heads are raised and thus brought into a withdrawal position. The defective sheets can then be removed easily by the machine operator.
Various sensor systems for monitoring sheets in printing presses are known in the prior art. For example, a light curtain which is described in German Patent Application DE 197 07 660 A1, corresponding to U.S. Pat. No. 5,944,431, is disposed below a reversing drum of a sheet-fed printing press for monitoring the sheet-reversal area. Transmitters and receivers of that sensor system are disposed on the drive and operating sides, on both sides of the reversing drum.
A sensor for monitoring the sheet run in a digital printing press is known from European Patent EP 2 562 107 B1, corresponding to U.S. Patent Application Publication US 2013/0050377.
On one hand, inaccuracies of the sensor systems can have the result that defective sheets are not detected. On the other hand, good sheets can be incorrectly detected as defective sheets.